Biofuel production is mainly done through transesterification. Transesterification is a catalytic reaction that allows producing biofuel (and glycerol as a byproduct) from oils (waste vegetable oil or pure plant oil) and alcohol. During the transesterification reaction, the ester exchanges the alkyl from the COO-R group with and alcohol, thus producing a new ester.
Pure plant oils are mainly composed of triglycerides (resulting from the glycerol alcohols functions esterification by three fat acid molecules). When the triglycerides react with alcohol (usually methanol), the three fat acid strings come apart from the glycerol skeleton to settle down on the alcohol, thus producing an oil ester (the biofuel for instance, hereafter noted FAME: Fatty Acid Methyl Esters).
The chemical equation of this reaction is as follow:

CH2-O-CO-R1

CH3-O-CO-R1

CH2-OH

I

catalyst

I

CH-O-CO-R2

+

3 CH3-OH

<=>

CH2-O-CO-R2

+

CH-OH

I

I

CH2-O-CO-R2

CH2-O-CO-R2

CH2-OH

Triglyceride

Methanol

FAME

Glycerol

This reaction can be catalyzed by an alkaline, acid or enzymatic catalyst. Only the first will be illustrated in this example.

The main characteristics of processes that imply an alkaline catalyst are:

-

Alcohol-oil molar ratio of 6 to 1

-

The conversion ratio of oil into methyl ester is 90% to 98% in 90 minutes.

-

The process is highly sensitive to the reactants purity: the presence of water into the reaction medium can lead to saponification (fat acids react with the alkaline catalyst to produce soap and water). This saponification not only leads to a catalyst deficiency, but the soap also takes part in emulsions formation, which generate difficulties to separate and purify biofuel.

-

The use of anhydride reactants is particularly important and is limiting for processes using waste vegetable oils. A pre-treatment step is required.

Modeling the processRigorous process simulation is today increasingly used to design and optimize biofuel production processes.
It also provides a starting point for advanced simulation of such process by presenting a set of unit operation modules and components with their physical properties.

Generally speaking, advanced simulation software like ProSimPlus enable pre-size equipment, run troubleshooting and debottlenecking analysis. Their ability to run many scenarios allows solving these types of problems within a reduced time and a minimum investment.

An example of biofuel production process was built with ProSimPlus. The process presented hereafter is using pure vegetable oils as raw material. Biofuel can be produced from different pure vegetable oils: colza, sunflower seed, soya and even waste vegetable oils. Taking into consideration its extensive industrial use, colza oil was selected in this example. The process involves reactors, distillation columns, extraction columns and components splitters.

This example can be used to analyze and understand the main areas of the process and shows one way to model these particular areas and their interconnections. Additional areas of investigation can also be the testing of new equipment configuration to enhance production yield and analysis of energy efficiency.
It is to be noted that this particular model is not intended to be used in equipment detailed design, manufacturing or even producing engineering documents without further review by a process engineer.